The present invention relates generally to the field of web bursting machines which separate sheets from webs having transverse lines of weakening across the web, and more particularly to machines of this nature which burst two webs simultaneously to maximize the sheet production rate of these machines.
It has long been well known to print various types of business forms and documents as a continuous web on which the individual forms or documents are delineated by transverse lines of weakening across the web. Typically such forms and documents are subject to certain types of preliminary operations prior to end use, such as mailing, which requires that the forms and documents be separated from each other to prepare them for these operations, such as stacking, collating, folding and inserting. A variety of machines have been developed and are commercially available which perform these, as well as other, preliminary operations in a continuous stream to maximize the output of the complete forms or documents processing system.
In a typical single web bursting machine, the bursting operation is achieved with the use of two sets or pairs of feed rollers, separated by a bursting element, usually in the form of a ball or cone. The upstream pair of rollers rotates at a predetermined speed to feed the web linearly at a predetermined speed along a guide mechanism toward the burst cone and then into the nip of the downstream pair of rollers which rotates at a higher speed than the upstream pair or at the same speed until the moment of burst, depending on the type of operation of the bursting machine. In one mode of operation, the downstream pair of rollers rotate continuously at a higher speed than the upstream pair rollers to maintain sufficient constant tension on the web to burst a sheet from the web at the instant that the transverse line of weakening is over the burst cone or ball. This mode of operation is not frequently used because the constant tension on the web tends to cause the web to burst prematurely or tear between the lines of weakening, which could be prevented to some degree by providing a relatively long distance between the upstream pair of rollers and the downstream pair of rollers, but this in turn tended to make the bursting machine inordinately large. Thus, bursting machines operating in this mode are not commercially popular. In the other mode of operation, both pairs of feed rollers operate at the same speed, but the downstream feed rollers are momentarily accelerated at the instant that the transverse line of weakening is over the burst cone to thereby apply sufficient tension to separate the sheet from the web. In still another mode of operation, again both pairs of feed rollers operate at the same speed, but the upstream feed rollers are momentarily decelerated or even stopped at the instant that the transverse line of weakening is over the burst cone, to thereby apply the same tension to the sheet to separate it from the web.
While this arrangement in any of these modes of operation worked very well in the variety of bursting machines then available, a significant drawback of these machines was that there is a limit to the speed at which these machines can operate due to certain practical considerations which is considerably below the rate at which other component machines in the sheet processing system can perform the specific operations for which they are intended. For example, in a typical processing system, a web of forms, which may be billing statements for a large number of bank customers, is fed to a bursting machine which separates the individual sheets of the statement of each customer from the web and feeds these sheets to a stacker, which stacks the proper number of sheets for the statement of each customer as determined by a suitable code printed on the sheets and which is read in or near the bursting machine. The stacked sheets are fed along a conveyer past a collating unit which inserts various types of materials, such as advertising flyers, etc., into the stack of forms, after which the stack is fed to a folding machine where the stack is folded to fit the mailing envelope into which the folded stack is inserted by an inserting machine. The filled envelope is then fed to a sealing machine which closes the envelope flap and seals it, after which it is fed to a mailing machine which prints a postage indicia thereon and finally stacks the envelope for mailing. A single machine which performs all of these functions sequentially is typically very large, e.g., 15 to 20 feet in length, is highly complex in construction with very sophisticated controls, and accordingly is normally very costly, often exceeding a hundred thousand dollars It is therefore important from a business profitability standpoint that machines of this nature operate at maximum efficiency.
As indicated above, one problem with single web bursters is that they cannot produce individual sheets from the web at the rate at which the component machines in the system can process the sheets. The major problem is that it is critical that the momentary tension that is applied to the web to cause a sheet to separate properly be applied at the exact instant that the transverse line of weakening in the web is directly adjacent to the burst ball or cone across which the web is drawn. There is a practical limit to the degree with which this can be controlled which is imposed by the nature of the devices used to effect this momentary tension. Traditional clutches, brakes and accelerating devices were quite slow. Present day stepping and brushless direct current motors provide much more accurate control and permit considerably higher operating speeds than was heretofore possible, but still cannot supply individual sheets at the rate at which the component machines in the system can handle them.
One solution known in the art has been to provide two webs which are fed through the bursting machine simultaneously, but with the sheets of one web offset longitudinally by one half pitch or sheet length from the other web, and from which sheets are separated alternately in either one of two modes of operation, either a single sheet from each web alternately, or a plurality of sheets successively from each web alternately. Obviously this arrangement greatly increased the efficiency and throughput rate of bursting machines, but introduced other problems, such as creep of one web with respect to the other, which resulted in misalignment of the two webs, thereby necessitating the use of two separate feeding and bursting components, one for each web. This in turn considerably increased the complexity and cost of the bursting machines, resulting in a further effort to obtain still greater efficiency and sheet throughput than was theretofore possible.
The major factor that limits the speed of operation of a dual web, dual burst mechanism machine, is that the webs are not fed through the machine at a constant velocity. Regardless of which mode of operation is utilized, i.e., single or plural sheets alternately, while one web is being advanced a sheet length to bring a transverse line of weakening to the burst cone, the other web is stationary. Thus, in the single sheet, alternate burst mode of operation, each web is stationary for the length of time required to move the other web one sheet length. In the plural sheet, alternate burst mode, each web is stationary for the length of time required to move the other web the length of the number of successive sheets to be burst from that web. Thus, for each bursting operation on one web, there is a certain amount of "down time" of the other web, which, in the totality of a particular web bursting and sheet processing operation, can accumulate sufficiently to amount to a very substantial loss of operating efficiency with resulting loss of profitability of the entire sheet processing system.